This program:
package domain
import cats.{Show, derived}
import cats.data.NonEmptyList
import cats.implicits._
case class Error(code: String, message: String)
case class Errors(errors: NonEmptyList[Error])
object ShowStuffApp {
// import NonEmptyList.catsDataShowForNonEmptyList
implicit val show: Show[Errors] = derived.semiauto.show
def main(args: Array[String]): Unit = {
val errors = Errors(NonEmptyList.one(Error("1234", "Could not process")))
val res = errors.show
println(res)
}
}
prints out:
Errors(errors = NonEmptyList(head = Error(code = 1234, message = Could not process), tail = Nil$()))
so it has auto-derived a Show instance for the NEL, rather than using the commented implicit one (which is in scope given the top-level imports); the docs at https://github.com/typelevel/kittens?tab=readme-ov-file#user-content-derive-show state that existing implicits are respected, but this does not seem to be the case here. Uncommenting the inner import has no effect. Ideas?
It's missing Show instance for Error class and NonEmptyList.catsDataShowForNonEmptyList is useless without it. If you add
implicit val showErrow: Show[Error] = derived.semiauto.show
to your code the println prints the following value:
Errors(errors = NonEmptyList(Error(code = 1234, message = Could not process)))